1. Field of the Invention
The present invention relates to a multiple-stage ladder type surface-acoustic-wave filter constructed by cascade-connecting one port surface-acoustic-wave resonators (these will be referred to as SAW resonators hereafter) in the form of a multiple-stage ladder.
2. Description of Related Art
As prior arts related to surface-acoustic-wave filters (these will be referred to as SAW filters hereafter) of this type, for example, those described in the following references exist.
REFERENCE 1: THE TRANSACTIONS OF THE INSTITUTE OF ELECTRONICS, INFORMATION AND COMMUNICATION ENGINEERS, J76-A [2] (1993-2) page. 245-252
REFERENCE 2: THE TRANSACTIONS OF THE INSTITUTE OF ELECTRONICS, INFORMATION AND COMMUNICATION ENGINEERS, J76-A [2] (1993-2) page. 236-244
FIG. 1 shows the structure of a conventional SAW resonator. FIG. 2 shows an equivalent circuit of the conventional SAW resonator shown in FIG. 1. This SAW resonator 10 has an interdigital transducer (this will be referred to as "IDT" hereafter) 11 and grating reflectors 12 and 13 installed on both sides of this IDT 11. These grating reflectors 12 and 13 are made of metal strips. This SAW resonator 10 is constructed, for example, by forming an IDT 11 and a pair of reflectors 12 and 13 made of an Al film on a 36.degree. Y-X-LiTaO.sub.3 single crystal substrate (this will be referred to as "LT substrate" hereafter).
The SAW resonator 10 is represented by an equivalent circuit in which a capacitance device C2 is parallel-connected with a linear circuit that is formed by serially connecting a reactance device L with a capacitance device C1.
FIG. 3 shows the configuration and principle of a one-stage constant K type filter in which the SAW resonator 10, which is described in the REFERENCE 1, shown in FIG. 1 is used. FIG. 4 shows the admittance characteristics of the SAW resonators 20 and 30 of the filter shown in FIG. 3 and the insertion loss characteristic of the filter. In the graph shown in the upper part of FIG. 4, the vertical axis represents the admittance. In the graph shown in the lower part of FIG. 4, the vertical axis represents the insertion loss (decibel: dB).
This one-stage constant K type filter is constructed as follows. The SAW resonator 10 shown in FIG. 1 is used to form a series-arm SAW resonator 20 and a shunt-arm SAW resonator 30. The series-arm SAW resonator 20 and shunt-arm SAW resonator 30 are then connected in the form of a ladder so that the series-arm SAW resonator 20 will generate an upper pole of attenuation and the shunt-arm SAW resonator 30 will generate a lower pole of attenuation. As a result of this configuration, a band pass filter is formed by setting the resonance frequency of the series-arm SAW resonator 20 equal to the anti-resonance frequency of the shunt-arm SAW resonator 30 based on the theory of constant K type filter.
FIG. 5 shows the configuration of a multiple-stage (4-stage) constant K type filter for explaining the principle of a multiple-stage (for example, 4-stage) constant K type filter in which the SAW resonator 10 described in the REFERENCE 2 and shown in FIG. 1 is used. FIG. 6 shows the insertion loss characteristics of the filter shown in FIG. 5. In FIG. 6, the horizontal axis represents the frequency and the vertical axis represents the insertion loss (dB). The multiple-stage constant K type SAW filter shown in FIG. 5 has a ladder network configuration formed by cascade-connecting multiple ladder stages. In this case, each of the ladder stages is made of a series-arm SAW resonator and a shunt-arm SAW resonator that are formed on a piezoelectric substrate.
Normally, a one-stage constant K type filter as shown in FIG. 3 does not provide a sufficient amount of attenuation. Therefore, in order to provide a sufficient amount of attenuation, one-stage constant K type filters are cascade-connected (for example, in 4-stages) to form a multiple-stage filter. When such a multiple-stage filter is formed, identically constructed stages are cascade-connected. Therefore, the resonance frequencies of the series-arm SAW resonators 20.sub.1 through 20.sub.4 and the resonance frequencies of the shunt-arm SAW resonators 30.sub.1 through 30.sub.4 are all set equal to each other. However in order to reduce the size of the filter, neighboring series-arm SAW resonators 20.sub.2 and 20.sub.3 or neighboring shunt-arm SAW resonators 30.sub.1 and 30.sub.2 or neighboring shunt-arm SAW resonators 30.sub.3 and 30.sub.4 are combined in some cases. Such neighboring shunt-arm SAW resonators or neighboring series-arm SAW resonators are then replaced with a different resonator having the same resonance frequency. In FIG. 6, the point indicated by A is an upper pole of attenuation formed by the series-arm SAW resonators 20.sub.1 through 20.sub.4. The point indicated by B is a lower pole of attenuation formed by the shunt-arm SAW resonators 30.sub.1 through 30.sub.4.
The conventional multiple stage constant K type ladder type SAW filter, however, has the following problems (1), (2), and (3).
(1) Normally, a one-stage constant K type ladder type SAW filter as shown in FIG. 3 does not provide a sufficient amount of attenuation. Therefore, in order to provide a sufficient amount of attenuation, one-stage constant K type filters are cascade-connected (for example, in 4-stages) to form a multiple-stage filter shown in FIG. 5. Therefore, the resonance frequencies of the series-arm SAW resonators 20.sub.1 through 20.sub.4 and the resonance frequencies of the shunt-arm SAW resonators 30.sub.1 through 30.sub.4 are all set equal to each other. Since all the series-arm SAW resonators 20.sub.1 through 20.sub.4 resonate at the same frequency, each of the series-arm SAW resonators 20.sub.1 through 20.sub.4 anti-resonates and generates the same upper pole of attenuation at the same frequency as shown in FIG. 6. Therefore, a sufficient amount of attenuation can be obtained in a narrow frequency band. However, it is difficult to obtain a wide frequency band in which a sufficient amount of attenuation is secured when a small number (for example four) of stages are used to form a multiple-stage cascade-connected filter. Hence, in order to secure a sufficient amount of attenuation over a wide frequency band, the number of stages used in the multiple-stage cascade-connected filter needs to be increased to a great extent. However, as the number of stages used in the multiple-stage cascade-connected filter is increased, the insertion loss in the pass band is also increased, which is a problem. PA0 (2) FIG. 7 shows the configuration and principle of a multiple-stage (for example, 4-stage) constant K type filter constructed by adding a pole of attenuation to the constant K type filter shown in FIG. 5. FIG. 8 shows the insertion loss characteristic of the filter shown in FIG. 7. In FIG. 8, the horizontal axis represents the frequency and the vertical axis represents the insertion loss (dB). The multiple-stage constant K type SAW filter shown in FIG. 7 also has a ladder network configuration formed by cascade-connecting multiple ladder stages. In this case also, each of the ladder stages is made of a series-arm SAW resonator and a shunt-arm SAW resonator that are formed on a piezoelectric substrate. PA0 (3) In the case of the band elimination type filter constructed using only series-arm SAW resonators described in the REFERENCE 2, the resonance frequencies of all the series-arm SAW resonators are changed. In this type of filter, the cramp capacitance of all the series-arm SAW resonators are serially connected in the pass band. As a result, the synthesized cramp capacitance becomes very small. Hence, unless the number of electrode fingers and aperture length of each of the series-arm SAW resonators are increased significantly, the insertion loss in the pass band increases. As a result, the chip size is increased and the insertion loss in the low frequency band of the pass band is also increased, which is a problem.
On the other hand, a sufficient amount of attenuation can be obtained over a wide frequency band without cascade-connecting any additional stage by reducing the cramp capacitance of each of the series-arm SAW resonators 20.sub.1 through 20.sub.4 or by reducing the Q of each of the shunt-arm SAW resonators 30.sub.1 through 30.sub.4 by increasing the cramp capacitance of each of the shunt-arm SAW resonators 30.sub. through 30.sub.4. However, if such a method is used, the insertion loss in the pass band also increases, which is a problem.
There is a method for increasing the amount of attenuation by serially adding poles of attenuation having different frequencies when the attenuation band is set in a higher frequency band than the pass band. Hence, a SAW resonator 21 for adding poles of attenuation is installed in the circuit shown in FIG. 21. In FIG. 8, the local minimum of the graph indicated by C is the upper pole of attenuation formed by the SAW resonator 21. It should be noted, however, that this additional upper pole of attenuation causes the characteristic of this filter in the high frequency side of the pass band to deteriorate as indicated by D in FIG. 8.
Therefore, when this method is used, not only the amount of insertion loss in the pass band increases as shown in FIG. 8 but also the number of required devices increases, causing the chip size to increase.